1. Field of the Invention (Technical Field)
The present invention is related to capacitors having low equivalent series resistance.
2. Background Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Wireless sensors and networks of wireless sensors will be used to 1) monitor the structural health of buildings, bridges and aircraft, etc.; 2) monitor the environment such as in domestic and commercial buildings, and military and homeland security installations; and 3) control industrial processes for increased autonomy, as well as for other tasks. These systems will find use in factory automation, process and environmental control, security, medicine, and condition-based maintenance, as well as in defense applications and intelligence gathering. Widespread use of wireless sensors will improve safety, increase security, lower heating, ventilation and cooling (HVAC) costs, and increase manufacturing efficiency.
Such wireless sensor systems will typically: 1) require numerous individual devices (known as nodes or motes) to provide comprehensive monitoring capability; 2) be located in inaccessible places and 3) require long intervals between scheduled maintenance. Periodic maintenance, such as replacing batteries, would clearly increase operating costs (often to prohibitive levels), and could be inconvenient, at best, if it required interruption of a continuous process. For some remote, hostile, or inaccessible locations, any maintenance may be impossible to perform. In the immediate future, energy management and improved battery technologies may mitigate some of these issues, but in the long term there is clearly a need to develop an energy source that can last years with little or no maintenance.
Miniaturized turbines and micro-fuel cells have been proposed as means of meeting long term energy delivery needs for wireless devices. While these systems exploit the high energy density of hydrocarbon fuels, for example, these systems are inherently limited by the need for a means of storing and delivering a consistent fuel supply, as well as advanced thermal management to safely remove waste heat. These challenges can be overcome; however, the plumbing and additional system engineering (also known as the balance of plant) adds considerably to the overall size and complexity of such systems.
There are additional challenges with micro-fuel cells. Most types are intended for hydrogen fuel, as protons are the simplest ion to send through the electrolyte. As hydrogen is not readily available, other fuels (e.g., hydrocarbons, methanol or formic acid, or natural gas) can be reformed with steam at high temperature (600° C.) to yield hydrogen and CO. These reformers again add engineering complexity and require extensive insulation for both safety and efficient operation. Furthermore, reported data for micro-fuel cells indicate maximum peak power densities on the order of 50 mW/cm2 but with a duration of less than 100 ms. These challenges ensure that combustion and micro-fuel cell power systems will be unable to meet the volumetric energy and power densities needed for severely volume-constrained applications.
Energy harvesters that garner ambient environmental energy (such as light, vibrations, etc.) and convert it to electrical energy are attractive solutions for wireless sensors as they do not need to be replaced, recharged or refueled. Of course, they are they do not function in the absence of ambient energy (analogous to solar cells at night), and so an energy harvesting power supply must be designed to include some kind of energy storage that can provide back-up power in such situations.
Storage of the energy generated is usually accomplished using conventional capacitors, which have very limited energy storage capability (E=½CV2, where the capacitance, C, is on the order of a few hundred microfarads at most, and V=3-5V). This approach leaves the system vulnerable to interruptions in the ambient energy source. Although batteries and or supercapacitors have been proposed as alternative storage devices, they have not been used successfully in the past. Conventional battery chargers, for example, will not operate at the low power levels delivered by energy harvesters, and, besides, even in they could, they would waste too much of the input energy. Further, no existing system discloses the use of optimum energy storage elements for different functions (e.g. back-up power, pulse power, etc.).
Finally, a major challenge that faces wireless sensor nodes is the asymmetry of the power demands of sensing, processing, communication and sleep functions—on the order of four orders of magnitude. Because communication functions draw relatively high power levels (typically from tens to a few hundred milliwatts), wireless sensor nodes are designed to communicate infrequently (for example, once a minute to once an hour), reverting to a low-power sleep state to preserve battery life. In order to meet high power communications loads, the usual approach is to design a power source large enough to handle the highest power load. Unfortunately, energy harvesting devices and batteries typically have low power densities, and so power sources are typically oversized for most of the life of the system.
There is, therefore, a need for a simple and compact system which combines energy generation via harvesting of ambient energy sources with energy storage to provide back-up power, and deliver high power pulses as needed.
Electric double layer capacitors (EDLCs) typically comprise two porous electrodes that are electronically isolated from each other by a porous separator. Both the separator and the electrodes are impregnated with an electrolytic solution. This allows ionic current to flow between the electrodes while preventing electronic current from shorting the cell. External connection to the electrodes is made via metallic current collectors. When an electric potential is applied across the two electrodes in a double layer capacitor, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Energy is stored at the interface between the electrodes and the electrolyte in the so-called Electric Double Layer. This is accomplished by absorption of the charge species themselves or by realignment of the dipoles of the solvent molecule. The absorbed charge is held in the region by the opposite charges in the solid electrode.
The use of carbon electrodes in electrochemical capacitors is a preferred feature of this technology because carbon has a low atomic weight and carbon electrodes can be fabricated with very high surface areas. As capacitance, C, is proportional to the surface area, A, and inversely proportional to the dielectric thickness, d, (i.e., C=∈A/d, where ∈ is the permittivity), EDLCs can realize very high capacitance values (from approximately 100 mF up to approximately 3000F). Fabrication of double layer capacitors with carbon electrodes is well known in the art; see U.S. Pat. Nos. 2,800,616, and 3,648,126.
A major problem in many carbon electrode capacitors, including double layer capacitors, is that the performance of the capacitor is often limited because of the high internal resistance of the carbon electrodes. This high internal resistance may be due to several factors, including the contact resistance of the electrodes with a current collector, the intrinsic resistance of the electrode due to internal carbon-carbon contacts, the resistance of the electrolyte solution and the resistance due to the separator. This high resistance translates to large ohmic losses in the capacitor during the charging and discharge phases, which losses further adversely affect the characteristic RC time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time. There is thus a need in the art for lowering the internal resistance, and hence the time constant, of double layer capacitors.
So that supercapacitors can deliver their energy quickly (that is, at high power), for example pulsed power delivery, it is important that they have a low equivalent series resistance (ESR) to minimize the voltage drop that occurs with high currents. The greater the ESR the greater the voltage drop will be when a load is applied to the charged capacitor. The voltage drop under load is critical if the circuit that is being powered can only operate above a certain threshold voltage. If the voltage under load of the EDLC drops below that threshold operating voltage of the electronic circuit it will not operate correctly if at all. Therefore EDLC devices with low ESR are desirable.
Several factors affect the ESR of an EDLC device including the interfacial contact resistance between the electrode and the current collector, interfacial charge transfer mechanisms between the electrolyte and the electrode, the interfacial contact resistance of the electrode and the separator, the contact resistance of the particles that make up an electrode, the effective bulk resistivity of the electrode, and the conductivity of the electrolyte and how it moves through the separator and electrodes.
There are four materials considerations that can help reduce the ESR of a supercapacitor.                The separator is a porous, electronically insulating membrane designed to allow ionic transfer but electrically separate the carbon electrodes. Because the mobility of ions (μ) through this membrane is restricted, the conductivity is reduced.        The nature of the electrolyte, including chemical species (z), and concentration (n) affects the ESR. Furthermore, the movement of electrolyte ions in the tortuous inter-particle porosity in the electrodes is also restricted, and so the nature of the pore distribution in the electrodes can also affect ESR values.        The activated carbon electrodes provide the means of charge transfer from the electrolyte-electrode interface to the current collector. The intrinsic resistivity of carbon and the particle-particle contact resistance contribute to this component.        The interface between the carbon electrodes and the current collectors is important. It is well known that the contact resistance between two materials with resistivity values ρ1 and ρ2 is:        
  R  =            (                        ρ          1                +                  ρ          2                    )        ⁢          (                        1                      4            ⁢                                                  ⁢            π            ⁢                                                  ⁢            a                          +                              3            ⁢                                                  ⁢            π                                32            ⁢                                                  ⁢            nl                              )      where n is the number of contacting asperities, a is the average radius of contacting asperities and 2l is the average center to center distance between the asperities [Greenwood, J. A., Br. J. Appl. Phys., 17, 1621-1632, (1966)]. Therefore, to minimize the contact resistance between two given materials, one must maximize n, 2l and a.
Various fabrication techniques for reducing the internal resistance of carbon composite electrodes have been disclosed over the recent years. For example, Yoshida et al. (U.S. Pat. No. 5,150,283) disclose a method of fabricating a aluminum/carbon composite electrode by depositing carbon powder and other electrical conductivity-improving agents on an aluminum substrate. Another related approach is disclosed in U.S. Pat. No. 4,597,028 (Yoshida et al.) which teaches that the incorporation of metals such as aluminum into carbon fiber electrodes can be accomplished through weaving metallic fibers into carbon fiber preforms. U.S. Pat. No. 4,562,511, (Nishino et al.) describes yet another approach where the carbon fiber is dipped into an aqueous solution such that a layer of a conductive metal oxide, and preferably a transition metal oxide, is formed in the pores of the carbon fibers. Nishino et al. also discloses the formation of metal oxides, such as tin oxide or indium oxide by vapor deposition. Still another related method is disclosed in U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963 (Tatarchuk et al.). These disclosures demonstrate that metal fibers can be interwoven with the carbon preform and sintered to create a structurally stable conductive matrix which may be used as a composite electrode. The Tatarchuk et al. patents also teach a process that reduces the electrical resistance in the composite electrode by reducing the number of carbon-carbon contacts, which current must flow through to reach the metal conductor. This approach works well if stainless steel or nickel fibers are used as the metal. However, this approach has not been successful when aluminum fibers are used because of the formation of aluminum carbide during the sintering or heating of the composite electrode.
The use of aluminum in fabrication processes of double layer capacitors is important because aluminum is the optimum metal in terms of cost, availability and performance. For example, with an aluminum/carbon composite electrode, in a double layer capacitor with a nonaqueous electrolyte, it is quite possible to achieve an operating voltage approaching 3.0 volts. However, with nickel or stainless steel in lieu of aluminum, the operating voltage must be reduced to less than 2.0 volts. Other metals, including noble metals, such as platinum or silver, or transition metals such as titanium or tantalum, may also be used up to 3.0V in non-aqueous systems with similar improvements.
Related designs of double layer capacitors are also discussed in U.S. Pat. No. 4,438,481, issued to Phillips, et al.; U.S. Pat. No. 4,597,028 issued to Yoshida, et al.; U.S. Pat. No. 4,709,303 issued to Fujiwara, et al.; U.S. Pat. No. 4,725,927, issued to Morimoto; and U.S. Pat. No. 5,136,472, issued to Tsuchiya, et al. Another area of great concern in the fabrication of double layer capacitors is concerned with the method of fabricating the current collector plate and adhering the current collector plate to the electrode. This is important because the interface between the electrode and the current collector plate is another source of internal resistance of the double layer capacitor. The Nishino et al. patent (U.S. Pat. No. 4,562,511) suggests plasma spraying of molten metals such as aluminum onto one side of the polarizable electrode thereby forming an appropriate layer which, if thick enough, acts as the current collector. This patent further considers alternative techniques for bonding and/or forming the current collector including arc-spraying, vacuum deposition, sputtering, non-electrolytic plating, and use of conductive paints. The Tatarchuk et al. patents (U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963) show the bonding of a metal foil current collector to the electrode by sinter bonding the metal foil to the electrode element. U.S. Pat. No. 5,142,451 (Kurabayashi et al.) discloses a method of bonding of the current collector to the electrode by a hot curing process such that the material of the current collectors enter the pores of the electrode elements. U.S. Pat. No. 5,099,398 (Kurabayashi et al.) discloses a method of bonding of the current collector to the electrode by chemically bonding a thin film collector such that some of the material of the current collectors enter the pores of the electrode elements. This patent further discloses some other conventional methods of bonding the current collector to the electrode including the use of electrically conducting adhesives and bonding under pressure and heat. Still other related art concerned with the method of fabricating and adhering current collector plates can be found in U.S. Pat. Nos. 5,065,286; 5,072,335; 5,072,336; 5,072,337; and 5,121,301 issued to Kurabayashi et al.
Carbon cloth provides a flexible electrode. However, it has a large amount of void volume due to the nature of the weave. It also tends to shed conductive lint and unravel at the edges. This can lead to a high incidence of short circuits during cell assembly. In addition, obtaining a low resistance electrical contact to the cloth requires special techniques. Farahmandi et al. (U.S. Pat. Nos. 7,116,545, 7,090,706, 6,842,330, 6,804,108, 6,627,252, 6,585,152, 6,451,073, 6,449,139, 6,233,135, 6,094,788, 6,059,847, 5,907,472, 5,862,035, 5,777,428, 5,621,607) disclose a method for plasma spraying aluminum onto a carbon cloth electrode such that the aluminum penetrates the fiber tows of the cloth, thereby decreasing the resistance of the electrode. The aluminum/carbon composite electrodes are bonded to aluminum foil current collectors via application of pressure at elevated temperatures (360° C.-600° C.). In these and the other cases above, a liquid or vapor phase metal is sprayed onto the surface of a nonconforming electrode, such as a carbon cloth. In other words, the metal conforms to the electrode cloth before it is solidified. The purpose for this is primarily to decrease the internal electrode resistance.
Laminating or sintering the electrode to the current collector is another method of reducing ESR. An adhesive may be used to adhere the electrode to the current collector with pressure so that the electrode is fixed to the current collector. Also, thermoplastics may be used as binders in the electrode so that when the electrode is pressed onto the current collector with heat and pressure the thermoplastic will melt and stick to the current collector. Polytetrafluorethylene (PTFE) may also be used to adhere the electrode to the current collector. When colloidal suspensions of PTFE are used as the binder for active carbon electrodes, the rolled flattened electrode sheet may be rolled simultaneously with a sheet of metal (aluminum) foil to which it will adhere.
Currently available EDLC devices packaged in the coin cell configuration which are not manufactured using the typically expensive processes discussed above have relatively high ESR values compared to wound, or “jelly roll” devices. Coin cell packages are desirable in applications were energy density is important because they tend to have a higher energy density than wound devices. However, coin cell electric double layer capacitors typically use thicker (greater than approximately 200 μm) electrodes than typically found in capacitors made using wound electrodes. The increased electrode thickness contributes to a higher ESR. Wound EDLC's typically have much lower ESR values than coin cells, in part because of their thinner (less than approximately 200 μm) electrodes.
Currently available commercial supercapacitors made in a coin cell type package claim ESR values in the range of 30-200 ohms, the majority being above 75 ohms. Supercapacitors with these values are usually constructed of two separate supercapacitor coin cells connected and packaged in series and have rated voltages of 5.5V. Because resistance is additive (RT=R1+R2) with devices in series the ESR value for an individual supercapacitor making up the series pair will be half of this range, or 15-100 ohms. In contrast, jelly-roll capacitors typically have an ESR ranging from 0.03-0.5 ohms.
Thus there exists a need for coin cell capacitors having reduced internal electrode-current collector resistance, and thus lower ESR, that are amenable to low-cost manufacturing.